Throughout the world there are vast reserves of hydrocarbons in the form of oil shales. Oil shales are sedimentary inorganic materials that contain appreciable organic material in the form of high molecular weight polymers. The inorganic portion of the oil shale is a marlstone-type sedimentary rock. Most of the organic material is present as kerogen, a solid, high molecular weight, three dimensional polymer which is insoluble in conventional organic solvents. Usually the naturally-occurring oil shales contain a small amount of a benzene-soluble organic material which is referred to as bitumen.
The most extensive oil shale deposits in the United States are the Devonian-Mississippian shales. The Green River formation of Colorado, Utah and Wyoming is a particularly rich deposit, and includes an area in excess of 16,000 square miles. The in-place reserves of the Green River formation alone exceed 3 trillion barrels. The Piceance Basin of Colorado represents nearly 85 percent of the Green River reserves.
A typical Green River Oil Shale is comprised of approximately 85 wt. percent mineral (inorganic) components, of which the carbonates are the predominate species, and lesser amounts of feldspars, quartz and clays are also present. The kerogen component represents essentially all of the organic material, and the elemental analysis is approximately 78% carbon, 10% hydrogen, 2% nitrogen, 1% sulfur and 9% oxygen.
Most of the methods for recovering hydrocarbon or organic material from oil shale materials involve mining the oil shale material, crushing it, and subjecting the crushed oil shale materials to thermal decomposition. The thermal decomposition of oil shale, i.e. pyrolysis or retorting, yields liquid, gases and solid (coke) products. The relative amounts of oil, gas and coke produced are controlled primarily by varying the parameters of temperature and time during the course of retorting the oil shale. Modern oil shale retorting processes operate at about 480.degree. C., (896.degree. F.) in order to maximize the yield of liquid hydrocarbon products.
Several major problems remain unsolved in the commercialization of the processes for recovering hydrocarbon from oil shale by retorting. The cost of mining the oil shale rock is excessive even in shallow deposits. One in situ retorting process has been described in the literature, where a rubbilized zone is created and a retorting process is applied in situ. This is applicable only to shallow deposits and yields are even less than surface retort yields because of the difficulty in controlling operating parameters. A substantial amount of the hydrocarbon component of the oil shale is consumed by combustion to generate the high temperatures needed for the pyrolysis reaction. The synthetic crude produced by retorting is very high in olefins and low in saturates and aromatics, and so a substantial amount of hydrogen must be added during refining to produce a good quality crude suitable for conventional refining. The hydrocarbon fraction which is produced in the gaseous state in the retorting process is greatly diluted by carbon dioxide resulting not only from the combustion of hydrocarbon portions of the oil shale, but also from thermal decomposition of the carbonate mineral fraction of the oil shale. Since dolomite and calcite are stable at temperatures far above the normal retorting temperatures, most of this carbon dioxide is derived from decomposition of dawsonite and nahcolite.
The state-of-the-art surface retorting method only recovers about 56% of the kerogen as a useful product. Because of this, as well as the other problems discussed above, there is essentially no commercial production of synthetic crude oil from oil shale in the United States at the present time despite the enormous reserves represented by the oil shale deposits. It can be seen from the foregoing discussion that there is a substantial, unfulfilled need for a new process for recovering useful hydrocarbon products from oil shale by an in situ process which eliminates the need to mine the oil shale rock, or reduces the cost for recovering the oil, or increases the percent of kerogen converted to useful product, or preferably accomplishes many or all of these objectives.
In my U.S. Pat. No. 4,566,964 which issued Jan. 28, 1986 for a Method of Recovery of Hydrocarbon from Oil Shale, there is disclosed a process in which mined oil shale material is crushed and ground to a predetermined fineness and then contacted with a free oxygen-containing gas such as air in a solvent for the organic fragments which are extracted from the polymeric kerogen components. The preferred solvents are naphthalene, tetralin and phenanthracene. The solvent which contains the organic fractions separated from the kerogen polymer is then separated into solvent and the organic fraction by sublimation. The residual solids comprising mineral and unoxidized kerogen is then subjected to a bake-off to recover additional organic materials from kerogen. This process permits recovery of hydrocarbon from oil shale materials using much lower temperatures than conventional retorting techniques, and accomplishes an increased recovery efficiency. The method does require that the oil shale material first be removed from the subterranean formation, and that it be crushed and ground to a relatively fine consistency. The cost of this mining, crushing and grinding operation is substantial and it is an object of the present invention to eliminate the need to mine the oil shale in order to reduce the overall cost of extracting hydrocarbon from the oil shale deposits.